In recent years, there have been demands for transmission systems that enable optical transmission of a large volume of data with an optical network. As transmission systems for optical networks, for example, multicarrier modulation systems, such as the discrete multi-tone (DMT) modulation system, are known. The DMT-modulation system is one of multicarrier transmission technique based on the orthogonal frequency division multiplexing (OFDM) technology. The DMT modulation system is a technique in which data is allocated to multiple subcarriers (SC) at different frequencies and the sets of data allocated to the respective SCs are modulated based on the modulation levels and the signal power quotas to transmit the data at high speed as a DMT signal.
When the system starts, an optical transmitting device employing the DMT modulation system performs a negotiation using a probe signal with an optical transmitting device that is a partner with which the optical transmitting device communicates and, acquires the reception characteristics based on the result of the negotiation, and sets the acquired reception characteristics for the transmission characteristics. According to the transmission characteristics, which are set, the optical transmitting device determines the modulation levels (the number of bits) and the signal power quotas corresponding to respective SCs. The optical transmitting device then modulates sets of data respectively allocated to the SCs based on the determined modulation levels and signal power quotas corresponding to the respective SCs to generate a DMT signal.
FIG. 19 is an explanatory view illustrating exemplary transmission characteristics relating to an optical DMT signal in which a dip occurs. Compared to the ideal transmission characteristics X11, the transmission characteristics X12 represented in FIG. 1 deteriorate as the allocated frequency increases because of the limitation on the band due to the frequency characteristics of the device in the optical transmitting device. Furthermore, in addition to the frequency characteristics of the device, noise, and deterioration in nonlinearity, a dip D occurs due to interaction between the chirp occurring in an optical sending device and wavelength dispersion on the optical transmission path, which deteriorates the transmission characteristics X13.
The frequency at which the dip D occurs can be calculated by assigning the modulation factor for the optical DMT signal, the chirp of the optical sending device, the wavelength of the optical DMT signal, the wavelength dispersion on the optical transmission path, and the fiber length (transmission distance) of the optical transmission path to Equation (1).
                              I          R                =                  m          ⁢                                    1              +                              α                2                                              ⁢                                                cos              ⁡                              (                                                                            π                      ⁢                                                                                          ⁢                                              λ                        2                                            ⁢                                              DLf                                                                                                                                  ⁢                          2                                                                                      c                                    +                                                            tan                                              -                        1                                                              ⁡                                          (                      α                      )                                                                      )                                                                                    (        1        )            where m is a modulation factor, α is a chirp, λ is a wavelength, D is a dispersion, and L is a fiber length.
FIG. 20 is an explanatory view illustrating exemplary occurrence of a dip relating to the optical DMT signal according to each transmission distance. Even in the same optical DMT signal, as illustrated in FIG. 20, a dip occurs at different frequencies according to the transmission distances of 10 km, 20 km, 40 km, and 80 km of single mode fiber (SMF) and the dip significantly lowers the relative intensity significantly, which deteriorates the transmission characteristics. This puts a limitation on the transmission capacity and transmission distance of the transmission characteristics.
Japanese Patent No. 4575703
Japanese Patent No. 5523582
FIG. 21 is an explanatory view illustrating exemplary transmission characteristics (transmission capacity to transmission distance) of an optical DMT signal. The transmission characteristics X14 represented in FIG. 21 are the transmission characteristics of an optical DMT signal. According to the transmission characteristics X14, while the transmission capacity of the transmission characteristics X14 is 130 Gbps when the transmission distance is 0 km (back to back: B to B), a dip occurs when the transmission distance is 10 km, which deteriorates the transmission capacity to 100 Gbps. In this way, the transmission characteristics of the optical DMIT signal deteriorates due to the occurrence of dip resulting from interaction between, for example, the chirp of the optical sending unit and the wavelength dispersion that accumulates on the optical path.